Certain methods of applying protective sheeting of polymer material to a pipeline are employed in pipeline construction, in particular the construction of underwater pipelines to be laid on the bed of a body of water, and to which the following description refers purely by way of example. Underwater pipelines are normally constructed by joining the facing free ends of adjacent pipes aligned along a longitudinal axis to define a cutback; and winding protective sheeting about the cutback and said end portions. Certain pipelines are composed of pipes joined to one another to cover distances of hundreds of kilometers. The pipes are of normally 12-meter standard length and of relatively large diameter ranging from 0.2 to 1.5 meters. Each pipe comprises a metal cylinder; and a polymer coating for protecting the metal cylinder. The opposite free ends of each pipe have no coating, so the metal cylinders can be welded to one another. The pipes may be joined at land-based installations, or on laying vessels which also provide for laying the pipeline as the pipeline is constructed.
The joining operation comprises welding the metal cylinders together, normally with a number of weld passes; and coating the cutback. Once an annular weld bead is formed between two adjacent metal cylinders, the cutback extends astride the annular weld bead, along an uncoated portion. In other words, the cutback is substantially defined by the free ends of the pipes, extends axially between two end portions of the protective coatings, and must be coated with a protective coating to prevent corrosion.
Coating the unprotected portion along the cutback is known as ‘Field Joint Coating’, and comprises winding thick, 2-10 mm thick, protective sheeting about the cutback. The protecting sheeting is extruded, and is simultaneously wound about the cutback as the protective sheeting is formed. This method is described in Applicant's PCT Patent Application No. WO 2008/071773 and in EP Patent Application No. 1,985,909, PCT Patent Application No. WO 2010/049353, and PCT Patent Application No. WO 2011/033176. In the methods described in these documents, the protective sheeting is applied by an extrusion die mounted on a carriage which runs along an annular path about the longitudinal axis of the pipeline, and the polymer material is plastified by a plastifying device located close to the pipeline and either connectable selectively to the extrusion die, or connected to the extrusion die by a hose or semirigid pipe.
The protective sheeting is also applied to the two end portions of respective pre-existing coatings, and is pressed onto the pipeline to ensure the protective sheeting adheres properly.
When field joint coating, it is essential to seal the protective sheeting to the end portions of the pre-existing coatings to prevent infiltration and form a seamless coating along the entire pipeline.
To seal the pre-existing coatings to the protective sheeting, PCT Patent Application No. WO 2007/079720 A1 proposes heating the metal cylinder by an induction heater, so the metal cylinder transmits heat by conduction to the protective sheeting and the end portions.
The above method calls for a relative enormous amount of thermal energy, which is dispersed and may endanger adhesion of the end portions of the pre-existing coating to the metal cylinder.